A fuel cell is a power generation system for producing electrical energy through the electrochemical redox reaction of an oxidant and a fuel such as hydrogen, or a hydrocarbon-based material such as methanol, ethanol, natural gas, and the like.
The fuel cell can be classified as a phosphoric acid type, a molten carbonate type, a solid oxide type, a polymer electrolyte type, or an alkaline type, depending on the kind of electrolyte used. Although each of these different types of fuel cells basically operates in accordance with the same principles, they may differ from one another in the type of fuel, operating temperature, catalyst, and/or the electrolyte used.
Recently, polymer electrolyte membrane fuel cells (PEMFCS) have been developed. They have power characteristics that are superior to conventional fuel cells, as well as lower operating temperatures and faster start and response characteristics. Because of this, PEMFCs have a wide range of applications such as mobile power sources for automobiles, distributed power sources for houses and public buildings, and for small electric sources for electronic devices.
A PEMFC is essentially composed of an electricity generating element, a reformer, and a fuel supplier. The electricity generating element forms a body of the PEMFC, and the fuel supplier provides the fuel stored in the fuel tank to the reformer. The reformer reforms the fuel to generate the hydrogen gas and supplies the hydrogen gas to the electricity generating element. Accordingly, in the PEMFC, fuel stored in the fuel tank is pumped to the reformer through a fuel pump and then the reformer reforms the fuel to generate hydrogen gas. The hydrogen gas and an oxidant are supplied to an electricity generating element through respective pumps. The electricity generating element generates electrical energy through the electrochemical reaction of the hydrogen gas and the oxidant.
The fuel cell may be a direct oxidation fuel cell (DOFC) in which liquid fuel is directly introduced to the electricity generating element. The direct oxidation fuel cell can omit the reformer which is essential for the polymer electrolyte fuel cell. Direct oxidation fuel cells may include a direct methanol fuel cell.
According to the above-mentioned fuel cell system, the electricity generating element for substantially generating the electricity has a structure in which several or many unit cells consisting of a membrane electrode assembly (MEA) and a separator (or referred to as a “bipolar plate”) are stacked. The membrane electrode assembly is composed of an anode (referred to as a “fuel electrode” or “oxidation electrode”) and a cathode (referred as an “air electrode” or a “reduction electrode”) separated by the polymer electrolyte membrane. The separators work as passageways for supplying the fuel and the oxidant required for the reaction to the anode and the cathode, respectively, and also work as conductors for serially connecting the anode and the cathode in the membrane-electrode assembly.
The electrochemical oxidation reaction of the fuel occurs at the anode, and the electrochemical reduction reaction of oxygen occurs at the cathode. Due to movement of the electrons generated by the reactions, electricity, heat, and water can be collectively produced.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore it should be understood that the above information may contain information that does not form the prior art that is already known in this country to a person or ordinary skill in the art.